Finding the wattage of motherboard

There is no connection between the wattage of the PSU and the wattage of the mobo...The only wattage ratings on the mobo is for the CPU(i.e., The TDP of the CPU).
The only thing that you would have to check with the PSU and the mobo is that whether the PSU has the required 24Pin connector, which connects to the mobo and a 4/ 8 Pin CPU power connector...

The real issue will be the wattage demands of the entire system you plan on building. Typically a motherboard built today might draw about 15 watts to support on board devices etc. It's the rest of your components which will determine the real wattage requirements. The biggest users of wattage will be the CPU and Video Card. If the video is embedded in the motherboard such as IGP (internal graphics port) the wattage draw will be greater than the typical 15 watts. The case with the 500 watt power supply may be adequite for a $500-800 system particularly if its a recognized brand. Its most likely not adequite for a newer Crossfire or SLI Video Gaming system, 2 video cards linked together with a high end processor. It is still enough for alot of users and could support a good single graphics solution system.